Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
Crosstalk is a serious problem in image sensors. There are three components to crosstalk: a) optical crosstalk, b) spectral crosstalk, and c) electrical crosstalk. Optical crosstalk is caused by the diffraction and/or scattering of light off of metal lines and at interfaces between the dielectric layers within the metal stack that overlays a CIS array. Spectral crosstalk results from the finite (nonzero) transmittance of color filters to wavelengths outside their target pass band, such as the finite transmittance of green and blue wavelengths through a red filter.
One form of electrical crosstalk is lateral drift of photo-generated charge carriers created deep in the semiconductor epitaxial layers (e.g., photogenerated electrons). As these photo-generated charge carriers rise, they can drift laterally and end up collected in the photodiode (“PD”) region of a neighboring pixel. Blooming is another form of electrical crosstalk characterized by the lateral diffusion of charge carriers when a PD region becomes full or saturated with charge carriers. Blooming is most commonly experienced in high luminous environments. Photo carriers that are generated near a saturated PD region will not be collected and therefore remain free to diffuse laterally into a neighboring pixel. Blooming results in the blurring of edges in still images and streaking in moving images. Both forms of electrical crosstalk are due to charge carriers generated in one pixel being collected by a neighboring pixel.
FIG. 1 illustrates a conventional CIS array 100 including three color pixels (red, green, and blue) that are susceptible to electrical crosstalk. Each PD region 105 of CIS array 100 is composed of a three-dimensional p-n junction having an N doped silicon middle surrounded by P doped silicon. Photons impinging on the PD regions 105 are absorbed in the silicon, creating photo-generated electron-hole pairs. Electron-hole pairs generated in the depletion region of the p-n junctions are separated effectively and the electrons are collected in the N type region for subsequent transfer through a transfer transistor during signal read out. However, electron-hole pairs generated outside of the depletion region are not separated as effectively and have a higher chance of diffusing to neighboring pixels, leading to reduced sensitivity and higher cross-talk. This phenomenon is more pronounced for small pixel sizes.